Non-Aqueous electrolyte secondary batteries

ABSTRACT

A non-aqueous secondary battery having an electrode group comprising a portion of exposed metal foil electrically connected to the positive electrode collector and covering the entire outer surface of the electrode group. Separator material is sandwiched between the foil and the negative electrode, which is positioned outwardly of the positive electrode, and separator material is also positioned between the foil covering of the electrode group and a negative polarity cell container. The positioning of the metal foil is such that if the battery is crushed, the separator material on the outermost side of the metal foil or between the metal foil and the negative electrode is broken first, so that the metal foil short-circuits with the inner wall of the cell container or the negative electrode, respectively.

This application is a U.S. National phase application of PCTINTERNATIONAL APPLICATION PCT/JP97/01040.

TECHNICAL FIELD

The present invention relates to a non-aqueous electrolyte secondarybattery having an electrode group composed of a thin positive electrodeand a thin negative electrode with separator material sandwichedtherebetween, and more particularly to its safety.

BACKGROUND ART

Conventionally, non-aqueous electrolyte secondary batteries use achalcogenide, such as an oxide, sulfide or selenide of a transitionmetal as positive active material. For example, manganese dioxide,molybdenum disulfide, titanium selenide, or metal lithium sheet is usedas a negative active material. An organic electrolyte composed of anorganic solvent solution of lithium salt is typically used as anon-aqueous electrolyte. Such batteries are typically referred to aslithium secondary batteries, and are aimed at producing batteries ofhigh voltage, large capacity, and high energy density. In such lithiumsecondary batteries, however, although an intercalation compound havingexcellent charging and discharging characteristics may have beenselected as the positive active material, the charging and dischargingcharacteristics of the negative electrode was not always excellent, andit was difficult to assure a long cycle life. Furthermore, accidentssuch as fire and rupture due to an internal short circuit were likely tooccur, raising serious safety concerns.

The metal lithium in the negative active material in this battery isdissolved as lithium ions in the organic electrolyte due to discharge.When charging, the dissolved lithium ions deposit on the surface of thenegative electrode as metal lithium, but all of them do not depositsmoothly as in the initial state. Some of them deposit as dendrites ormossy, active, metallic crystals. Such active metallic crystals reactwith the organic solvent in the electrolyte, causing their surface to becovered with a passivation film, making them inactive and unable tocontribute to discharge. Therefore, the negative electrode capacitydrops as the charging and discharging cycles are repeated. Accordingly,when manufacturing the cells, it was necessary to set the negativeelectrode capacity extremely larger than that of the positive electrode.Besides, active lithium metallic crystals are likely to form an internalshort circuit by penetrating through the separator and contacting withthe positive electrode. By such an internal short circuit, the cell maysuddenly generate hear, causing a cell rupture or accidental fire.

Accordingly, the so-called lithium ion secondary batteries using amaterial for intercalating and deintercalating lithium ions by chargingand discharging as the negative material have been proposed, intensivelyresearched and developed globally, and are now already in a practicalstage. The lithium ion secondary battery, as long as it is notovercharged, does not deposit active metallic lithium crystals on thenegative electrode surface when charging, and enhancing safety. Itsdemand is growing rapidly in recent years because it is superior to theconventional lithium secondary battery in high-rate charge and dischargecharacteristics and life cycle. In the lithium ion secondary battery,lithium is the active material, and thus, the battery may be regarded asa kind of lithium secondary battery. It can be distinguished, however,from the lithium secondary battery that uses conventional metalliclithium as the negative electrode.

As the positive active material of the lithium ion secondary battery, adouble oxide of lithium and a transition metal, such as LiCoO₂, LiNiO₂,LiMnO₂, or LiMn₂O₄ in discharged state, is used. As the negative activematerial, graphite or other carbon material similar in potential to themetallic lithium as charged is used in most systems, but in othersystems of low voltage operation, in part, a double oxide of lithium andtransition metal is used in the negative electrode.

When the lithium ion secondary battery is charged and discharged, thepositive active material can reversibly repeat deintercalation andintercalation of lithium ions, and the negative active material canreversibly repeat intercalation and deintercalation of lithium ions, sothat the cycle life is extremely long. Moreover, because of high voltageand/or large capacity, a battery of high energy density is provided.

However, these lithium ion secondary batteries, like the conventionallithium secondary batteries, employ organic electrolytes of relativelylow ionic conductivity. Accordingly, a thin positive electrode andnegative electrode are fabricated by thinly forming an active materiallayer or a mixture layer of active material and conductive agent on ametal foil of current collector. An electrode group is composed bysetting the positive electrode and negative electrode oppositely to eachother separated by a thin microporous polyolefin resin membraneseparator. By increasing the opposing surface areas of the positiveelectrode and negative electrode, a practical high-rate charge anddischarge characteristic is maintained to expand conformity to manyapplications. For example, the positive electrode and negativeelectrode, each piece in a thin and long strip form sandwiching aseparator therebetween, may be spirally wound or plaited like anaccordion, or a plurality of positive electrodes and negative electrodesmay be laminated alternately with a separator therebetween to form theelectrode group.

In these lithium ion secondary batteries, a separator capable of closingfine pores and thus decreasing the ion conductivity when raised to aspecified temperature is used to cut off current. Moreover, anelectronic protection circuit in the battery pack is used to controleach cell to prevent fatal deterioration due to overcharge andoverdischarge. Therefore, when used normally, safety is assured, but inabnormal use, it is hard to guarantee safety. For example, when abattery pack in a fully charged state is crushed by a strong externalforce, such as being run over by an automobile, or when overcharged dueto malfunction of the protection circuit as described above, theseparator in the cell may be broken, and the positive and negativeelectrodes are shorted. Such shorted electrodes generate heat by Jouleheat or reaction heat, and when the decomposition temperature of thepositive active material is achieved, active oxygen is generated. Theactive oxygen violently oxidizes the solvent in the organic electrolyteor the other material in the cell, causing a state of thermal runaway.As a result, the cell temperature rises sharply in an instant, possiblyleading to cell rupture or accidental fire. The risk of such accident isalso present when the charged battery pack is disposed of with commonhousehold refuse.

To prevent such accidents, usually, in each cell, a temperature fuse,PTC device, other temperature rise preventive means, and anexplosion-proof safety valve are provided, but may not be sufficient tocope with the sudden temperature rise due to a thermal runaway. It wastherefore proposed to provide a cell capable of preventing a sudden risein cell temperature, thus preventing cell rupture and accidental fire asexperienced hitherto when the positive electrode and negative electrodeare short-circuited, such as when the separator is broken due to thecell being crushed or overcharged. A typical example is disclosed inJapanese Laid-open Patent Application No. Hei8-153542, relating to alaminate electrode assembly (electrode group) comprising a positiveelectrode and a negative electrode, each comprising an active materiallayer at least on one side of a metal foil which is a collector,positioned opposite to each other with a separator therebetween. Theconfronting portions of the metal foils of the collector of the positiveelectrode and the negative electrode are exposed at least on one side,over at least one turn or one layer or more, with the separatortherebetween, in any one of the electrode group outermost portion,innermost portion, or intermediate portion.

In such cell a composition, when the side surface is pressed, the cellis crushed, the separator is torn, and the positive electrode andnegative electrode contact each other, the short-circuit current flowsselectively between the exposed metal foil portions of the collector ofthe positive electrode and the negative electrode, which are higher inelectronic conductivity than the active material layer, and the positiveand negative active materials in a charged state are discharged andconsumed in a short time, so that the cell temperature may not be raisedto a critical state. Moreover, in order to short-circuit securelybetween exposed portions of the metal foil of the positive and negativeelectrode collectors, this same publication also discloses means forselectively tearing the separator between the exposed portions of thepositive and negative metal foils by interposing a part made of a rigidor elastic body at least in one of the exposed portions of the positiveand negative electrode metal foils.

As a result of close studies of these proposed cell compositions, thepresent invention proposes a cell composition having an electrode groupfor selectively short-circuiting in a position of high electronicconductivity between the metal foil of the positive electrode collectorand the negative electrode, easily releasing heat in the cell withoutsacrificing the cell capacity or increasing the number of parts morethan necessary. By employing such a cell composition, it is intended topresent a non-aqueous secondary battery high in reliability and enhancedin safety, capable of securely preventing accidents such as rupture orfire, even in the event of the abnormality of crushing the cell.

DISCLOSURE OF INVENTION

The invention relates to a non-aqueous electrolyte secondary batterycomprising an electrode group composed by sandwiching a separatorbetween a thin positive electrode and a thin negative electrode. Eachelectrode comprises a metal foil, which is a collector, having a thincoating thereon, the coating comprising an active material layer or amixture layer of active material and conductive agent. The electrodegroup is configured such that the negative electrode is positionedoutwardly relative to the positive electrode, and a portion of exposedmetal foil, which is electrically connected to the positive electrodeand has no active material layer or no mixture layer of active materialand conductive agent thereon, covers the outer side of the negativeelectrode with a separator therebetween. The outermost side of theexposed metal foil is also covered with a separator. The electrode groupso constructed is put in a negative polarity cell container togetherwith non-aqueous electrolyte. Thus, the exposed metal foil connected tothe positive electrode collector covers the entire surface of the outerside of the electrode group, having one side facing the negativeelectrode and the other side facing the inner side wall of the negativepolarity cell container. Therefore, if the cell side surface is pushedby a strong external force and the cell is crushed, the separator on oneside or both sides of the metal foil of the positive electrode collectorpositioned outside of the electrode group is first broken, and the metalfoil for the positive electrode short-circuits with a least one of thenegative electrode and the inner wall of the negative polarity cellcontainer. The positive and negative materials in a charged state arethus discharged and consumed in a short time, and because theshort-circuit position is adjacent to the cell container, the heat isreleased easily, thereby preventing a sudden rise in cell temperature.As a result, cell rupture, fire or other accidents may be prevented, sothat the reliability and safety are successfully enhanced withoutincreasing the number of parts or sacrificing the cell capacity morethan necessary.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a sectional view of a cylindrical non-aqueous electrolytesecondary battery according to an embodiment of the invention.

FIG. 2 shows the positive electrode having exposed surfaces on bothsides of the metal foil of the collector over a sufficient length forcovering at least the outermost periphery of the electrode group in acylindrical cell in an embodiment of the invention.

FIG. 3 show comparative examples 1, 2, 3 of the positive electrode forcylindrical cell.

FIG. 4 shows a prior art of positive electrode for cylindrical cell.

FIG. 5 shows a cross-sectional diagram of an exemplary oval-shaped cellof the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to the drawings and table below, a preferred embodiment ofthe invention is described specifically below.

FIG. 1 is a sectional view of a lithium ion secondary cylindrical cell(overall height 70 mm, diameter 20 mm) in an embodiment of a non-aqueoussecondary battery of the invention.

FIG. 1, an electrode group is constructed by spirally winding a positiveelectrode 1 of 57 mm in width and 520 mm in length, and a negativeelectrode 2 of 59 mm in width, 550 mm in length, and 0.2 mm inthickness, with a separator 3 therebetween. The separator is made of amicroporous polypropylene membrane.

The positive electrode 1 is fabricated by first preparing a positiveelectrode paste by adding and mixing an artificial graphite as aconductive agent to an active material made of a double oxide (LiCoO₂)of lithium and cobalt prepared by baking a mixture of lithium carbonate(Li₂CO₃) and cobalto-cobaltic oxide (Co₃O₄) in air at 900° C. A 5 wt. %dispersion solution of polytetrafluoroethylene (PTFE) is added and mixedinto the paste as a binder. Both sides of an aluminum (Al) foil 1 a arecoated with this positive electrode paste, dried, and pressed byrolling, forming a mixture layer 1 b of active material and conductiveagent. According to the invention, a 57 mm portion of positive electrode1 (corresponding to the length of at least one periphery of the outercircumference of the spiral electrode group) has the Al foil of thecollector exposed without any mixture layer of the active material andconductive agent on either side, as shown in FIG. 2.

The portion of foil 1 a coated with mixture layer 1 b may be considereda “first foil portion” whereas the exposed portion of foil 1 a may beconsidered a “second foil portion” that is electrically connected thefirst foil portion. As shown in FIGS. 2 and 3, the first and second foilportions may merely comprise portions of a single foil. A positiveelectrode lead piece 1 c is spot-welded to another exposed portion of Alfoil at the end of the positive electrode opposite Al foil exposedportion 1 a.

The negative electrode 2 is fabricated by mixing 5 wt. % ofstyrene-butadiene rubber as a binder to the active material, whichcomprises artificial graphite powder with an average particle size of 3μm. This negative electrode paste is then dispersed in carboxymethylcellulose (CMC) aqueous solution and applied to both sides of the copper(Cu) foil of the collector. The paste is then dried, and the electrodeis pressed by rolling, and cut. A negative electrode lead piece 2 c isspot welded to an exposed portion of Cu foil at one end of the cutnegative electrode 2.

The electrode group is then spirally wound, starting with separator 3 atthe winding core, and ending with at least one turn of the exposedportion 1 a of the positive electrode 1 covering the negative electrode2, which is on the outer circumference. Separator 3 is sandwichedbetween exposed foil portion 1 a and negative electrode 2 and alsocovers the outermost circumference of the exposed foil. The outsidediameter of the electrode group was 18 mm.

Afterwards, the upper surface and lower surface of the electrode groupare heated by hot air, and any portions of the separators 3 extendingpast the upper end and lower end of the electrode group are shrunk. Abottom insulating plate 4 is fitted and put in a cell container 5, and anegative electrode lead piece 2 c is spot welded to the inner bottomsurface of the cell container 5. An upper insulating plate 6 is mountedon the electrode group, a groove is cut in a specified position of theopening of the cell container 5, and a proper amount of non-aqueouselectrolyte is poured in. The non-aqueous electrolyte comprises 1 moleof lithium hexafluorophosphate (LiPF₆) dissolved in a mixed solvent ofethylene carbonate (EC) and diethyl carbonate (DEC) at volume ratio of1:1. 1 liter of organic electrolyte was prepared. Later, a terminalplate 7 a and a cover plate 7 b are crimped into one body, a gasket 7 cis fitted in the peripheral edge, and a positive electrode lead piece 1c is spot welded to the cover plate 7 b of the lower side of thisassembled cover. The assembled cover is fitted into the opening of thecell container 5, and the upper edge of the cell container 5 is curvedinward to seal. This embodiment is listed as cell A in the table below.

FIGS. 3A, B and C show comparative examples 1, 2 and 3 fabricated forconfirming the effect of the invention.

In comparative example 1, the length of the exposed portion of the Alfoil of positive electrode is 30 mm long, as compared with the length of57 mm in the exposed portion of the Al foil of the positive electrode ofthe invention. When the electrode group is composed by using thepositive electrode of the comparative example 1, the entire outercircumference of the electrode cannot be covered with the exposedportion of Al foil.

In comparative example 2, the positive electrode has an exposed portionof Al foil over a length of 57 mm at the winding core side of theelectrode group to which the positive electrode lead piece 1 c iswelded.

In comparative example 3, the positive electrode is 30 mm long in theexposed portion of the Al foil at the winding core side.

In the prior art shown in FIG. 4, there is no exposed portion of Alfoil, unlike the positive electrodes in the invention and in thecomparative examples.

Using the positive electrodes of comparative examples 1, 2, 3 and priorart, cells were fabricated by constructing the rest of the electrodegroup and the rest of the cell in same manner as cell A, with all thesame other parts, materials and methods. These cells are respectivelycalled B, C, D and E.

50 cells each of the fabricated cells A, B, C, D and E were charged for2.0 hours at 20° C., at constant current of 800 mA and constant voltageof 4.2V per cell. All charged cells were presented for a crushing test,and the number of cells breaking out in fire was counted. The resultsare summarized in Table 1. In the crushing test, a cylindrical metal rodof 10 mm in diameter of steel or the like was placed so as to bevertical to the cell axial line, at the cell outer wall side of themiddle of the overall height of each cell, and the cell was pressed andcrushed by a pressing machine until the diameter became half thediameter prior to crushing.

TABLE 1 Cell Positive electrode Number of cells breaking out in fire AInvention 0/50 B Comparative example 1 3/50 C Comparative example 2 4/50D Comparative example 3 3/50 E Prior art 7/50

As is clear from the results in Table 1, fire took place in 7 of cells Eof the prior art, while no fire was caused in cells A of the invention.In cells B, C and D using the positive electrodes having exposedportions of Al foil in the outer circumference or winding core portionof the electrode group, the number of cells breaking out fire was abouthalf that of cells E, but was not zero.

In cell A of the invention, as mentioned above, the entire outercircumference of the electrode group is wound at least one turn in metalfoil that conducts electrically with the positive electrode, withseparator on either side of the metal foil. In other words, the metalfoil for the positive electrode is wound at least one turn around theouter circumference of the electrode group, with one side facing thenegative electrode, with separator therebetween, and the other sidefacing the inner wall of the negative polarity cell container, withseparator therebetween. When the cell is crushed in this state, firstthe separator closer to the outer side in the cell is torn, and themetal foil of the positive electrode collector selectivelyshort-circuits with at least one of the cell container or the negativeelectrode of the outermost circumference. As mentioned above, since theouter circumference of the electrode group is covered by at least oneturn of the exposed metal foil conducting electrically to the positiveelectrode, if any position of the cell side surface is crushed bypressing, short-circuit occurs as stated above, and no fire results inthe cell.

By contrast, in the cell B, as mentioned above, since the entire outercircumference of the electrode group is not covered with the exposedportion of the Al foil of the positive electrode, the metal rod used inthe crushing test may not always press the exposed portion of the Alfoil. This explains why fires cannot be completely eliminated in thecell B.

In the cells having the exposed portion of Al foil of the positiveelectrode collector at the winding core side of the electrode group, asin the cells C and D, the separator adjacent the exposed portion of themetal foil is not always torn. When the positive and negative activematerials contact directly with each other due to breakage of separator,thermal runaway may occur in certain cells. The electrode groupcompositions in C and D are advantageous in that the cell capacity ishardly sacrificed, as disclosed in Japanese Laid-open Patent ApplicationNo. Hei8-153542, but accidental cell fires cannot be eliminatedcompletely, which is a problem in the aspect of reliability. The samepublication proposed separator breakage parts made of metal barsinserted into the winding core. These cells were evaluated in a crushingtest, and it was confirmed that the cell fires could be nearlyeliminated. In such cell compositions, however, since the number ofparts increases, the cell manufacturing cost and weight are increased,and hence it is not an optimal solution.

As mentioned above, according to the proposal disclosed in JapaneseLaid-open Patent Publication Application Hei8-153542, the cell isconstructed by disposing the confronting portions of the metal foils ofthe positive electrode and negative electrode collectors, having atleast one side exposed and separator therebetween, over a length of oneturn or one layer, in any one of the outermost side, innermost side, andthe intermediate portion of the electrode group. In the presentinvention, the exposed portion of metal foil is only on the positiveelectrode, the entire outer surface of the electrode group has thenegative electrode positioned at the outer side and covered with theexposed metal foil portion of the positive electrode with separatortherebetween, the outermost side of the foil is wrapped with separator,and the electrode group is put in a negative polarity cell container. Inthe present invention, the exposed portion is not provided in the metalfoil of the negative electrode collector because the active material inthe charged state of the lithium ion secondary battery and lithiumsecondary battery is highly conductive. For example, the carbon materialin which the lithium is inserted, expressed as C6Li, and the metalliclithium are both highly conductive. If short-circuited, an active oxygengeneration source is not obtained unless there is direct contact withthe positive active material. Therefore, in the cell composition of theinvention, the capacity of negative electrode or cell is not sacrificed.It is a benefit of the invention that the entire outer surface of theelectrode group is covered with a very thin metal foil connected to thepositive electrode collector and with a separator. In such cellcomposition, the cell capacity is slightly sacrificed, but the higherreliability in cell crushing and improved safety are considered moreimportant.

So far, a cylindrical cell has been described as the embodiment. Theinvention, however, is not limited to the cylindrical cell alone. It mayalso be applied to an oval cell using an oval section electrode group.Such an oval section electrode group is formed using a thin and longpositive electrode and a thin and long negative electrode configured ina flat plate in the winding core portion with a separator therebetween,and then plaiting down in one direction and winding up, An exemplaryoval cell, as shown in cross-section in FIG. 5, comprises electrodegroup 100 comprising positive electrode 1, negative electrode 2, andseparator material 3 therebetween. Central winding core 101 isconfigured in the shape of a flat plate. Exposed metal foil portion 1 a(also expressed as “the second metal foil”) is electrically connected toouter circumferential end 102 of positive electrode 1 (also expressed as“the first metal foil,” which is the portion of foil 1 a covered bymixture layer 1 b). Negative polarity cell container 103 has an ovalgeometry.

The oval cell is also similar to a prismatic cell using an electrodegroup made by plaiting down a thin and long positive electrode and thinand long negative electrode in an accordion form with a separatortherebetween, or a prismatic cell using an electrode group comprising aplurality of positive electrodes and negative electrodes alternatelylaminated with separators therebetween. Of the cells of these shapes,however, such as in the prismatic cell, where the electrode platecomprises the positive electrode and negative electrode plaited into anaccordion form with separator therebetween, the exposed portion of metalfoil must be provided at both ends of the positive electrode. In thecase of the electrode group composed by alternately laminating aplurality of positive electrodes and negative electrodes with separatorstherebetween, one more negative electrode is used than positiveelectrodes. The negative electrodes are thus positioned at the outersides, and two metal foils connected to the positive electrodecollectors are disposed at the outer side as dummy plates, with aseparator between each foil and the respective negative electrode. Theoutermost sides of the foil are also wrapped with separator. In thiscase, of course, the negative electrodes are connected electrically inparallel, and the positive electrode and dummy plates are connectedelectrically in parallel.

In the embodiment described herein, the active material was LiCoO2 inthe positive electrode, and carbon in the negative electrode, but theinvention is not limited to these systems alone. For example, as thepositive active material, a double oxide of lithium and transition metalexpressed as LiMO₂ or LiM₂O₄, where M is selected from the groupconsisting of Mn, Fe, Co, and Ni, may be used. As the negative activematerial, metallic lithium, Nb, Ti, and other transition metal oxidesmay be used as an alternative to carbon.

In the embodiment described herein, a microporous polypropylene membraneis used as the separator. Depending on the purpose, however, themembrane separator may be made of polyolefin such as polyethylene ormixture of polyethylene and polypropylene.

The non-aqueous electrolyte of the invention is not limited to theorganic electrolyte. The technology may be sufficiently applied topolymer solid electrolyte, too.

What is claimed is:
 1. A non-aqueous electrolyte secondary batterycomprising: an electrode group comprising: a positive electrode and anegative electrode having a separator material sandwiched therebetween,said positive electrode comprising a first conductive metal foil portioncomprising a coating of an active material or a mixture of an activematerial and a conductive agent, and a second conductive metal foilportion electrically connected to said positive electrode, wherein anoutermost portion of the negative electrode is positioned outwardly ofan outermost portion of the positive electrode, said second metal foilportion is positioned outwardly of the negative electrode outermostportion, and said separator material is positioned between the secondmetal foil portion and the negative electrode outermost portion andcovers an outermost side of the second metal foil portion; and anegative polarity cell container in which said electrode group is housedtogether with a non-aqueous electrolyte.
 2. The non-aqueous electrolytesecondary battery according to claim 1, wherein: the electrode groupcomprises said positive electrode and said negative electrode spirallycoiled together with said separator material therebetween, wherein saidsecond metal foil portion is electrically connected to an outercircumferential end of the positive electrode, the negative electrodeoutermost portion comprises a first outer circumference that is entirelycovered with the second metal foil portion that comprises a second outercircumference, said separator material positioned between said firstouter circumference and second outer circumference and covering saidsecond outer circumference.
 3. The non-aqueous electrolyte secondarybattery according to claim 1, wherein said electrode group has an ovalcross-section with said positive electrode, said negative electrode, andsaid separator material therebetween, configured in a flat plate in acentral winding core; in which said second metal foil portion iselectrically connected to an outer circumferential end of the positiveelectrode and wherein the negative polarity cell container has an ovalgeometry.
 4. A non-aqueous electrolyte secondary battery of claim 1,wherein the first metal foil portion that comprises the positiveelectrode has an exposed portion thereof without said coating, saidexposed portion defining said second metal foil portion.
 5. Thenon-aqueous electrolyte secondary battery of claim 2, wherein the firstmetal foil portion that comprises the positive electrode collector hasan exposed portion thereof without said coating, said exposed portiondefining the second metal foil portion.
 6. The non-aqueous electrolytesecondary battery of claim 3, wherein the first metal foil portion thatcomprises the positive electrode collector has an exposed portionthereof without said coating, said exposed portion defining the secondmetal foil portion.
 7. The non-aqueous electrolyte secondary battery ofclaim 1, wherein said electrode group is adapted to fail uponapplication of a crushing force by at least one failure mode selectedfrom the group consisting of: the separator material on the outermostside of said second metal foil portion breaking first so that the secondmetal foil portion short-circuits with an inner wall of said cellcontainer; the separator material between said second metal foil portionand said negative electrode breaking first so that the second metal foilportion short-circuits with said negative electrode.